Psoralens are the linear isomers of the furocoumarin family and they occur naturally in certain fruits and seeds, e.g., Ammi majus and Psoralea corylifolia. Extracts of these fruits and seeds have been used since ancient times as dermal sensitizing agents in the treatment of vitiligo. Topical application of psoralen extracts, followed by irradiation with light, results in a stimulation of melanin production, thus producing a dermal "tanning" effect.
In recent years, psoralens have been utilized in the photo-chemotherapy of psoriasis. In such treatment, psoralens are administered orally or topically to a patient. Subsequently, the skin is exposed to ultra-violet radiation. A high percentage of remissions of the disease occur after such treatment.
With increasing study of, and interest in, molecular biology, the psoralens have been investigated with respect to their ability to form covalent bonds with nucleic acids. Because of their planar structure, psoralens can intercalate between the base pairs in the double helix molecular structure of nucleic acids. Upon irradiation with light of the proper wavelength, the psoralens may form covalent bonds with pyrimidine nucleotides that occur as integral entities of nucleic acid strands. Achieving covalently bonded psoralen bridges or crosslinks between the nucleic acid strands of the double helix presents another tool for use in studying, in vivo, secondary structures of nucleic acids. In addition, the psoralens provide a means for inactivating viruses for the purpose of vaccine production, and also as potential chemotherapeutic agents. The covalently bonded psoralens act as inhibitors of DNA replication and thus have the potential to slow down, or stop the replication process. The covalent bond can only be produced in a two step process by first intercalating the psoralen into the nucleic acid helix, and secondly, by exposing those sites to electromagnetic radiation. Thus, it is immediately apparent that the covalent bonding can be controlled both temporally and spacially.
It will be also apparent that crosslinking can only occur for those psoralen molecules that are present at the right place at the right time, i.e., a psoralen molecule must have intercalated in the correct position at the exact moment radiant energy arrives at that site. The presence of a psoralen molecule at the proper position is dependent upon the solubility of the psoralen in aqueous solution and upon the dissociation constant for the non-covalent binding of the psoralen to nucleic acid. Thus, the higher the solubility, the greater number of molecules in the surrounding liquid medium available to binding sites. Similarly, the lower the dissociation constant, the greater the number of psoralens occupying a potential binding site at any moment in time. The dissociation constant, K.sub.D, for non-covalent binding of the psoralen to the nucleic acid is defined by the expression: EQU K.sub.D = (P) (S) /(PS)
where (P) is the concentration of free psoralen, (S) is the concentration of unoccupied binding sites where each base pair on a nucleic acid is considered to be a binding site, and (PS) is the concentration of psoralen-bound sites.